Exhaust gas recirculation (EGR) systems reduce automotive vehicle engine emissions by recirculating exhaust gases from the engine exhaust manifold into the engine combustion chambers to be re-burned. The re-burning results in more complete fuel combustion and fewer hydrocarbon emissions.
EGR systems of this type may be either electronic or electro-pneumatic. Electronic EGR systems include electronic control units that control the rate of exhaust gas recirculation by sending an electric control signal directly to an electrically-actuated EGR valve. In electro-pneumatic systems the electronic control unit sends its control signal to an electro-pneumatic solenoid valve that sends corresponding pneumatic signals to a pneumatic EGR valve. The electronic control units for both types of system calculate the optimum recirculation rate based on a given set of operating variables.
Electrically-actuated EGR valves sometimes use solenoids to control valve position. An electronic control unit controls valve position in a solenoid EGR valve of this type by varying the voltage or current of a non-oscillatory electrical input signal or by modulating the pulse-width of a fixed-frequency modulated (PWM) signal.
Solenoid EGR valves include inductive coils that generate magnetic fields when energized. The magnetic field of a typical solenoid valve drives a ferromagnetic armature valve between open and closed positions to meter exhaust gas flow. In a system using a non-oscillatory control signal, the exhaust gas flow rate is proportional to the current passing through the coil. In PWM-type systems, the valve opens and closes at the frequency of the electrical input signal with the exhaust gas flow rate being proportional to the signal pulse width.
The inductive coil in a solenoid EGR valve includes windings of electrically conductive wire, typically copper. The coil wire resistance changes with temperature. The supply voltage, e.g., battery voltage, used to derive the pulse train or non-oscillatory drive signal for the coil is relatively temperature-independent. Thus, any increase in coil wire resistance will result in a proportional decrease in electric current passing through the coil. Likewise, any decrease in coil wire resistance will result in a proportional increase in electric current passing through the coil. Changes in current through the coil change the magnetic field strength. Changes in magnetic field strength change the exhaust gas flow rate through the EGR valve. Therefore, as the temperature of the coil changes, the exhaust gas flow rate changes accordingly.
It is desirable for a solenoid EGR valve to include some means to compensate for changes in coil resistance that result from temperature changes. Current solenoid EGR valves, such as those disclosed in U.S. Pat. Nos. 5,094,218; 4,961,413 and 4,805,582 do not compensate for temperature changes.
The prior art does, however, include EGR systems having temperature-compensated electro-pneumatic converters with proportional solenoid valves that regulate vacuum pressure to vacuum-actuated EGR valves. For example, U.S. Pat. No. 4,522,371 to Fox et al., issued Jun. 11, 1985, (the Fox et al. patent) discloses an electro-pneumatic converter with a proportional solenoid valve that is electronically-controlled and compensates for temperature changes by modifying the control signal. The Fox et al. system includes a proportional solenoid valve with an inductive coil that, when energized, produces a magnetic field. An armature in the form of an annular magnetic closure member is disposed adjacent the coil and is movable under the influence of the magnetic field to adjust the vacuum pressure output of the solenoid valve. The inductive coil has a coil resistance value that varies with temperature. The magnetic field is controlled by a pulse-width-modulated control signal from an electronic control unit. In response to elevated ambient temperatures, the control unit increases the control signal duty cycle to compensate for the changed temperature condition (see column 8, lines 17-19).
To compensate for temperature-induced coil resistance changes by modifying the control signal in this way, an electro-pneumatic converter must include at least one remote sensor to measure ambient air temperature. A microprocessor or other logic device must receive inputs from the temperature sensor and be programmed to change the duty cycle or otherwise adjust the control signal it sends to the proportional solenoid valve to maintain the output vacuum pressure at a predetermined optimum value for a given set of operating variables.
Designing and manufacturing EGR systems that compensate for temperature changes by modifying the control signal involves considerable time and expense. For example, the addition of a temperature sensor requires the purchase of the sensor and connecting wire, wiring harness modifications and involves additional steps in an assembly-line process. In addition, a microprocessor must be purchased and programmed or an existing electronic control unit must be modified to process information from the temperature sensor.
An example of an electro-pneumatic EGR system that compensates for temperature without modifying the control signal is disclosed in U.S. Ser. No. 08/425,402, (the '402 application), currently pending in the U.S. Patent and Trademark Office and assigned to the assignee of this invention. The electro-pneumatic converter disclosed in the '402 application includes an inductive coil that generates a magnetic field when energized. An electronic control unit energizes the coil with a fixed-frequency pulse-width modulated signal. The periodic magnetic field drives a ferromagnetic armature valve open and closed--alternately admitting then closing-off a flow of atmospheric-pressure air that is used to alter the vacuum pressure output to the EGR valve. The vacuum pressure is dependent upon the amount of current flowing through the coil during each pulse. The coil resistance is temperature-dependent. To hold the coil current constant over temperature, the coil is connected in series with a thermistor having a temperature coefficient of resistance that is opposite that of the coil. A temperature-stable resistor is connected across the thermistor to modify its temperature-response curve to more closely offset that of the coil. This system achieves temperature compensation without modifying the control signal to the coil but requires a vacuum source and an electro-pneumatic solenoid valve that converts electrical signals from the control unit into pneumatic signals for operating the EGR valve.
What is needed is an EGR system that compensates for temperature changes without modifying the control signal, that includes fewer components, and that does not require a vacuum source to operate its EGR valve.